Access Denied trying to change io_scene_fbx/__init__.py
There are several places blender looks for python scripts/addons:

MS-Windows
LOCAL
.\2.76\.

USER
C:\Documents and Settings\$USERNAME\AppData\Roaming\Blender Foundation\Blender\2.76\

SYSTEM
C:\Documents and Settings\All Users\AppData\Roaming\Blender Foundation\Blender\2.76\


Generally the system path isn't writable for a standard user, but naturally this depends on the platform and your specific configuration. Try copying the script to the user path (you may get warnings about duplicate addons).
The local path applies to versions of blender installed though an archive extracted anywhere on your system, as opposed to an installer or package. (e.g. for a local install on windows, the .zip download can be used)